Process for chromosomal integration and DNA sequence replacement in clostridia

ABSTRACT

The present invention is related to a new method for replacing or deleting DNA sequences in Clostridia, with high efficiency, easy to perform and applicable at an industrial level. This method is useful to modify several genetic loci in Clostridia in a routine manner. This method is based on a replicative vector carrying at least two marker genes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No. 11/737,025, filed Apr. 18, 2007; which claims priority to PCT/EP2006/066997, filed Oct. 10, 2006. Each of these applications are hereby incorporated by reference herewith in their entirety.

BACKGROUND OF THE INVENTION

Clostridia are gram positive, anaerobic and low GC bacteria that are widely used in industry for their capacities to produce solvents, in particular butanol, ethanol and acetone, but also diols like 1,3 propanediol, organic acids like acetic, butyric or lactic acid and vaccines.

Construction of recombinant Clostridia is an important part of the development in the field. Clostridium strains are genetically modified in order to improve their industrial capabilities.

To perform these modifications, homologous recombination is the most used technique in all kinds of organisms. Transformation and homologous recombination in several microorganisms have been extensively described in the art. See for example (Datsenko and Wanner; PNAS, 2000) and (Fabret et al., Molecular Microbiology, 2002).

Clostridia are not naturally transformable and currently available methods for their transformation are inefficient and do not permit the introduction of multiple mutations. This has hampered industrial developments in this field.

Clostridia commonly produce extracellular DNAses and restriction enzymes that degrade foreign DNA before and after introduction into the cells for transformation. Classic methods based on the introduction of PCR fragments that work well in many microorganisms such as E. coli or yeast, are not feasible in these organisms, since the extracellular and intracellular half life of the DNA construct to be recombined is too short and recombination efficiency is generally low. In other organisms these difficulties have been circumvented by using vectors that replicate in the host thus increasing the likelihood of the recombination event. Nevertheless after the recombination event the vector that now carries the intact target DNA sequence has to be eliminated. This problem was solved in Lactococcus lactis (Biswas et al, J. Bacteriol., 1993) by using temperature-sensitive replicons that can be eliminated at the non-permissive temperature. No vectors with these characteristics are currently available for Clostridia. Therefore construction of mutants in Clostridia has so far been very laborious and often unsuccessful.

Inactivation of genes in Clostridia were reported in the following articles (see table 1).

TABLE 1 Strain Genotype Reference Clostridium acetobutylicum buk-, MLS^(R) Green et al., 1996 PJC4BK Clostridium acetobutylicum pta-, MLS^(R) Green et al., 1996 PJC4PTA Clostridium acetobutylicum aad-, MLS^(R) Green and PJC4AAD Bennett, 1996 Clostridium perfringens Δ cpe, CatP Sarker et al., 1999 SM101 and F4969 Clostridium perfringens Strain 13 Δ luxS Ohtani et al., 2002 Clostridium acetobutylicum SKO1 ΔspoA, MLS^(R) Harris et al., 2002 Clostridium perfringens Type A Δ spo0A Huang et al., 2004 Clostridium perfringens SM101 ccpA-, CatP Varga et al., 2004 Clostridium acetobutylicum ΔSpoIIE, WO 2006/007530 ATCC 824 buk- buk-, CatP

Gene inactivation was so far performed in Clostridia by transforming with circular DNA that could not replicate in the target strains. Since DNAses and DNA restriction endonucleases present in Clostridia rapidly degrade the introduced DNA, and generally the recombination frequency in this genus is not very high, the obtention of mutants has been very laborious.

In addition, the so far described recombinant strains (see above) are all resistant to MLS or chloramphenicol and the corresponding marker genes can not be removed after the recombination event has occurred. This limits the number of possible recombinations to the number of available resistance markers in these bacteria to a maximum of 3. Furthermore, for the industrial use of these bacteria, it might be useful to have markerless strains in order to avoid the release of antibiotic resistance genes into fermentation media.

Moreover, some of these strains that have been obtained by single recombination events have the disadvantage that they are not stable if cultured without any selection pressure.

Consequently, there is still a need in the state of the art for a method for the transformation of Clostridia with high efficiency, with an easy step of selection of recombinant strains, that allows successive DNA sequence replacements in the same strain, leading to recombinant Clostridia that are genetically stable and markerless.

The present invention is related to a new method for replacing or deleting DNA sequences in Clostridia, easy to perform and applicable at an industrial level. This method is useful to modify several genetic loci in Clostridia in a routine manner.

This method is based on a replicative vector useful for the transformation of Clostridia with high efficiency.

An unlimited number of mutations can be introduced into the genome with this new method, by eliminating resistance cassettes from the genome and reusing them in successive rounds of DNA sequence replacement.

Efficient introduction of multiple mutations into Clostridia should enable industry to improve existing industrial strains and to develop new processes.

DESCRIPTION OF THE INVENTION

The invention provides a process for the replacement of a target DNA sequence by homologous recombination in Clostridia, comprising:

-   -   Transforming said strain with a vector comprising:         -   an origin of replication permitting its replication in             Clostridia, and         -   a replacement cassette comprising a first marker gene             surrounded by two sequences homologous to selected regions             around the target DNA sequence, allowing the recombination             of the cassette, and         -   a second marker gene,     -   selecting strains having integrated in their genome said         cassette, that express the first marker gene,     -   selecting strains having eliminated said vector, that do not         express the second marker gene.

All molecular biology techniques used for realizing the invention are fully described in “Molecular cloning: a laboratory manual”, 2^(nd) edition, Cold Spring Harbor Laboratory Press, 1989, by Sambrook, Fritsch and Maniatis.

Used in the context of the present invention, the term “replacement” of a target DNA sequence means that a different sequence than the original one is introduced at the locus of the target DNA sequence.

According to the invention a DNA sequence is defined as a gene, or an intergenic sequence. Both can comprise promoter or regulatory sequences.

The expression “target DNA sequence” means any gene, intergenic region, promoter or regulatory sequence of interest chosen by the man skilled in the art. It means in particular genes coding for proteins of interest, for example enzymes involved in the cellular metabolism.

The substituted/inserted DNA sequence may be coding or not. It may be a mutated sequence of the target gene, promoter or regulatory sequence and/or a marker such as an antibiotic resistance gene or a color generating enzyme. It may be longer or shorter than the replaced sequence, depending on the distance separating the two homologous regions.

Due to the insertion, the expression of the target gene is usually perturbed, partially or completely suppressed, or increased. The replacement of the target DNA sequence with a sequence close to the original one, but comprising mutations, leads to the actual expression of a mutated protein, promoter or regulatory sequence.

If the replacement of the target DNA sequence gives as a result a total elimination of said DNA sequence, the gene is qualified as “deleted”.

The expression “homologous recombination” refers to the event of substitution of a segment of DNA by another one that possesses identical regions (homologous) or nearly so. This event is also called DNA crossover.

The term “transformation” refers to the incorporation of exogenous nucleic acid by a cell, this acquisition of new genes being transitory (if the vector carrying genes is cured) or permanent (in the case the exogenous DNA is integrated chromosomally).

The term “vector” refers to an extra-chromosomal element carrying genes or cassettes, that is usually in the form of a circular double-stranded DNA molecules, but may be a single strand DNA molecule, too. Both terms “vector” and “plasmid” are used indifferently.

The vector according to the invention is a replicative vector. It comprises at least one origin of replication, and preferentially several replicative origins, allowing it to be functional in different species.

In particular a preferred vector may comprise two replicative origins:

-   -   Ori, functional in E. coli,     -   RepL from pIM13 issued of B. subtilis, functional in Clostridia         (Mermelstein et al, Biotechnology, 1992).

The expression “sequences homologous to a target DNA sequence” refers to sequences with high sequence similarity with selected regions of the target sequence.

The term “marker gene” refers to a sequence coding for a marker protein under the control of regulatory elements functional in Clostridia. Such proteins are well known in the art. For example the man skilled in the art may use an antibiotic resistance gene, a fluorescent or colored marker, or a marker of auxotrophy. Examples of useful marker genes will be given below.

After the recombination event has occurred, the vector now carries the intact target DNA sequence and consequently has to be eliminated. The elimination of a replicative vector generally happens with successive cultures of clones, followed by negative or positive selection of clones having eliminated this vector. The elimination of the vector can also be an active step of the process, with the use of endonucleases that specifically cleaves DNA sequences present in the vector. Once the vector is cured, strains do not express the second marker gene anymore, and can be selected on this characteristic.

In a particular embodiment of the invention, the second marker gene is an antibiotic resistance gene. Among the useful antibiotic resistance genes, the man skilled in the art would know which is the most appropriate. For example the following genes may be used: CatP gene, giving the resistance to chloramphenicol and thiamphenicol, or MLS^(R) gene, giving resistance to erythromycin.

In a preferred embodiment of the invention, the second marker gene is a counter-selectable marker gene.

A counter-selectable marker is a gene whose presence is lethal to the host organism under certain circumstances, such as the presence of its cognate substrate. Counter-selectable markers can be used as a positive selection for the loss of the plasmid.

Preferentially the counter-selectable marker gene is a gene that restores the activity of an absent or deleted non-essential endogenous gene.

The most-used counterselectable markers are the genes that confer sucrose, streptomycin, or fusaric acid sensitivity. They have been used to construct mutants or vaccine strains in several bacterial strains. For details see the review by Reyrat et at., 1998, Infection and Immunity. Counterselectable markers that can be used in Clostridia include genes giving sensitivity to 5-fluoro-uracile (5-FU), gamma-glutamyl hydrazide (GBS) or 8-aza-2,6-diaminopurine (8ADP).

In a preferred embodiment, the counter-selectable marker is the upp gene, which encodes uracil phosphoribosyl-transferase that promotes transformation of 5-fluoro-uracile (5-FU) to a toxic product. Cells having Upp activity can not grow on a 5-FU medium.

The use of this counter-selectable marker is particularly useful when the transformed Clostridia are Δupp, and consequently are able to grow on a medium comprising 5-FU before the transformation and after the elimination of the vector. Strains having eliminated the vector can be positively selected.

Suppressor mutants that may arise in the upp gene in the presence of 5-FU, can sometimes lead to false assumptions with respect to the loss of the plasmid. In a preferred embodiment of the invention, the vector comprises furthermore a third marker, preferentially an antibiotic resistance gene that permits a second selection of strains sensitive to the antibiotic. This negative selection may be used in addition to the positive selection based on the upp gene.

In a preferred embodiment of the invention, the vector is eliminated by digestion with endonucleases after the recombination event has occurred. Preferentially, the vector harbors DNA sequences that are recognized by restriction endonucleases and that are at the same time absent from the genome of the Clostridium species used. Therefore the vector is specifically destroyed without loss of integrity of the Clostridium genome.

Restriction endonucleases are enzymes that cleave DNA molecules at the location of specific base sequences, the restriction endonuclease site. The expert in the field will be able to determine which restriction endonuclease site is absent from the genome of the Clostridium strain of interest. Possible restriction endonucleases that may be applied for C. acetobutylicum are AscI, FseI, NotI, SfiI, SrfI. In another embodiment meganucleases, which recognize large (12-45 bp) DNA target sites, such as I-SceI, HO or I-Crel may be used.

In a preferred embodiment of the invention the Clostridium strain to be transformed harbors on its genome at least one endonuclease encoding gene, which recognizes the restriction endonuclease site that is present on the vector. Optionally, the restriction endonuclease expression is under the control of an inducible promoter.

An inducible promoter is a DNA element that permits the conditional expression of a target sequence by adding the corresponding inducer. For example, an inducible promoter system in Clostridium that is known to the expert in the field is described in Girbal et al., 2003, Appl. Env. Microbiol. 69:4985-8.

After the recombination event has occurred, and before the screening of strains that have eliminated the vector, the expression of the restriction endonuclease may be induced. The restriction endonuclease will cleave the vector present in the Clostridia leading to its elimination.

Optionally the restriction endonuclease encoding gene can be inserted on the genome before the introduction of the vector into the strain.

In another embodiment of the invention the restriction endonuclease encoding gene may also increase the frequency of recombination before the elimination of the plasmid by increasing the amount of linear DNA in the cells that is known to recombine better than circular DNA.

In a particular embodiment of the invention, the first marker gene is an antibiotic resistance gene introduced in the middle of the replacement cassette.

In a specific embodiment of the invention, this first marker gene may be removed from the genome of the transformed Clostridium strains. In particular the first marker gene may be surrounded by two recombinase target sites, and then be eliminated by action of a recombinase, after the homologous recombination event has occurred.

In an advantageous embodiment of the invention, the recombinase is expressed by a second vector carrying the corresponding gene, said vector being introduced into the Clostridia by transformation.

Preferentially, recombinase target sites are FRT sequences. FLP recombinase from Saccharomyces cerevisiae is active at a particular 34 base pair DNA sequence, termed the FRT sequence (for FLP recombinase target). When two of these FRT sites are present, the FLP enzyme creates double-stranded breaks in the DNA strands, exchanges the ends of the first FRT with those of the second target sequence, and then reattaches the exchanged strands. This process leads to deletion of the DNA which lies between the two sites.

In a specific way of performing the invention, sequences homologous to selected regions around the target DNA sequence may comprise mutations in up to 10% of the base pairs composing the DNA fragment used for the recombination event.

In an advantageous embodiment of the invention, the Clostridium strains to be transformed are deleted for the genes encoding restriction endonucleases. These strains present the advantage that they are readily transformable without any prior in vivo plasmid methylation.

In another advantageous embodiment of the invention, the Clostridium strains to be transformed are deleted for the genes encoding the extracellular DNAses.

In another embodiment of the invention, the Clostridia to be transformed are deleted for the upp gene.

In another advantageous embodiment of the invention the Clostridium strains to be transformed are chosen among Clostridium acetobutylicum, Clostridium bejeirinckii, Clostridium saccharoperbutylacetonicum, Clostridium butylicum, Clostridium butyricum, Clostridium perfringens, Clostridium tetani, Clostridium sporogenes, Clostridium thermocellum, Clostridium saccharolyticum (now Thermoanaerobacter saccharolyticum), Clostridium thermosulfurogenes (now Thermoanaerobacter thermosulfurigenes), Clostridium thermohydrosulfuricum (now Thermoanaerobacter ethanolicus).

The preferred Clostridium strain is Clostridium acetobutylicum.

In a preferred embodiment of the invention, the Clostridium acetobutylicum strain to be transformed is a strain Δ Cac15, deleted for the gene encoding for the restriction endonuclease Cac 824I. This strain presents the advantage that it is readily transformable without any prior in vivo plasmid methylation.

In another preferred embodiment of the invention, the Clostridium acetobutylicum strain to be transformed is a strain whose upp gene was deleted, termed Δ upp. As a consequence the transformation of the strain with a vector carrying the upp gene allows the selection of strains sensible to 5-FU medium, and then the positive selection of strains having lost the plasmid that are not sensible to 5-FU medium anymore.

The Clostridium acetobutylicum strain to be transformed can also be Δ Cac15 Δ upp.

The process is advantageously used for successive replacement of two or more target genes by homologous recombination in the same Clostridium strain.

The present invention also concerns a recombinant Clostridium strain susceptible to be obtained by the process according to the invention. Advantageously, the process according to the invention may be used to obtain recombinant Clostridium strains wherein the gene cac15 was deleted (strain Δcac15), wherein the gene upp was deleted (strain Δupp) and wherein both genes cac15 and upp were deleted (strain ΔCac15 Δupp).

The present invention also concerns the vector for transforming the strain, such as described above.

DESCRIPTION OF DRAWINGS

FIG. 1: Map of the pUC18-FRT-MLS2 vector

FIG. 2: Map of the pCons2-1 vector.

FIG. 3: Map of the pCIP2-1 vector

FIG. 4: Map of the pCons::UPP vector

FIG. 5: Map of the pCLF 1 vector

FIG. 6: Schematic representation of the deletion method for Clostridia.

The following steps are successively performed:

-   -   A—Amplification of two selected regions around the target DNA         sequence; 1, 2, 3, 4 represent PCR primers     -   B—Cloning of obtained PCR fragments into the cloning vector         pTOPO     -   C—Insertion of a marker at the StuI restriction site, present in         the PCR primers     -   D—Cloning of the replacement cassette at the BamHI site of pCONS         2.1: construction of pREP clo Y vector     -   E—Transformation of Clostridium with pREP cloY vector     -   F—Chromosomal integration of replacement cassette by double         crossover during subculture     -   G—Screening of clones with Ery^(R) and Thiam^(S) phenotypes     -   H—PCR analysis to check for gene replacement and plasmid loss

OR

-   -   D′—Cloning of the replacement cassette at the BamHI site of         pCONS::UPP: Construction of pREP clo Y:UPP vector     -   E′—Transformation of Clostridium Δupp with pREP cloY: UPP vector     -   F′—Chromosomal integration of replacement cassette by double         crossover during subculture     -   G′—Screening of clones with Ery^(R) and 5-FU^(R) (Thiam^(S))         phenotypes     -   H′—PCR analysis to check for gene replacement and plasmid loss.

EXAMPLES Example 1 Construction of Vectors

1.1 Construction of pUC18-FRT-MLS2

This plasmid contains an MLS^(r) gene functional in Clostridia and flanked by two FRT sites and two StuI sites and is useful for the construction of the replacement cassettes. Inverse polymerase chain reaction (IPCR) was performed with Pwo DNA polymerase with pKD4 as a template plasmid (Datsenko and Wanner, 2000) and oligonucleotides PKD4.1 and PKD4.2 as primers to amplify the plasmid region with the FRT sites but without the Km^(r) marker. This blunt end fragment was later ligated to the MLS^(r) gene obtained after a HindIII digestion of the pETSPO plasmid (Harris et al, 2002, J. Bacteriol) and Klenow treatment. The corresponding plasmid (pKD4-Ery1) was then used as a template to amplify the MLS^(r) gene flanked by two FRT sites and two StuI sites in a PCR reaction using the oligonucleotides FRT-MLSR-F and FRT-MLSR-R as primers and Pwo DNA polymerase. This fragment was directly cloned into the SmaI digested pUC18 to yield the pUC18-FRT-MLS2 plasmid (FIG. 1).

Pcr primers: PKD4.1 (SEQ ID N° 1): 5′-ctggcgccctgagtgcttgcggcagcgtgagggg-3′ PKD4.2 (SEQ ID N° 2): 5′-agcccggggatctcatgctggagttcttcgccc-3′ FRT-MLSR-F (SEQ ID N° 3): 5′-tacaggccttgagcgattgtgtaggctggagc-3′ FRT-MLSR-R (SEQ ID N° 4): 5′-aac

gggatgtaacgcactgagaagccc-3′

1.2 Construction of pCons 2.1

This plasmid contains a pIM13 origin of replication functional in Clostridia (rolling circle mechanism of replication), a catP gene conferring resistance to thiamphenicol and a unique BamHI site for the cloning of the replacement cassette. This plasmid was constructed in a two step procedure from the pETSPO plasmid (Harris et al, 2002, J. Bacteriol) to remove part of a polylinker and among others a BamHI and an EcoRI site. IPCR was performed with Pwo DNA polymerase with pETSPO as a template plasmid and oligonucleotides PCONSAccI and PCONSEcoRI as primers, and the PCR product was phosphorylated and ligated. After transformation of E. coli the pCons0 plasmid was obtained. This plasmid was then digested with BamHI to remove the spoOA cassette, the proper DNA fragment purified and ligated to yield plasmid pCons2-1. The map of pCons2-1 is given in FIG. 2.

PCR primers: PCONSAccI (SEQ ID N° 5): 5′-ccggggtaccgtcgacctgcagcc -3′ PCONSEcoRI (SEQ ID N° 6): 5′-gaattccgcgagctcggtacccggc -3′

1.3 Construction of the pCIP 2-1 Vector

In this construction the pIM13 origin of replication from pCons2-1 was replaced by the origin of replication of the pSOL1 plasmid, a plasmid having a θ mechanism of replication. For this purpose the origin of replication of pSOL1 was PCR amplified with Pwo DNA polymerase using total DNA of C. acetobutylicum as a template and oligonucleotides ori-3-D and ori-4-R as primers. This PCR product was cloned in the pCR-BluntII-TOPO and the resulting plasmid digested by EcoRI and the 2.2 kb fragment was purified. Similarly the pCons2.1 plasmid was digested by EcoRI and the 2.4 kb fragment purified and ligated to the 2.2 kb EcoRI fragment containing the origin of replication of pSOL1 to yield the plasmid pCIP2-1 (FIG. 3).

Pcr primers: ORI-3-D (SEQ ID N° 7): 5′-ccatcgatgggggtcatgcatcaatactatcccc-3′ ORI-4-R (SEQ ID N° 8): 5′-gcttccctgttttaatacctttcgg-3′

1.4 Construction of the pCons::upp Vector

The upp gene with its own ribosome bindind site (rbs) was cloned into pCons2.1 at the BcgI site just downstream of the CatP gene in order to construct an artificial operon with upp expressed under the control of the CatP promoter. In this way, no homologous regions were introduced that would allow chromosomal integration of the upp gene in the Δcac15Δupp strain in further deletion experiments, which use the upp gene as a counter selectable marker for plasmid loss.

The upp gene with its rbs was PCR (Pfu) amplified from genomic C. acetobutylicum DNA using oligonucleotides REP-UPP F et REP-UPP R as primers. The 664 bp PCR-product was digested by PvuII and cloned into pCons2.1 digested by BcgI and treated with T4 DNA Polymerase to blunt ends. In this way the pCons::UPP (see FIG. 4) replicative vector was obtained.

PCR primers: REP-UPP F (SEQ ID N° 9): 5′- aaaacagctgggaggaatgaaataatgagtaaagttacac-3′ REP-UPP R (SEQ ID N° 10): 5′- aaaacagctgttattttgtaccgaataatctatctccagc-3′

1.5 Construction of the pCLF1 Vector

The FLP1 gene of S. cerevisiae coding for the FLP recombinase was cloned in the pCons2.1 vector under the control of the promoter and RBS from the thiolase (thl) gene from C. acetobutylicum permitting high expression in this organism. The FLP1 gene was PCR (Pfu) amplified using the FLP1-D and FLP1-R oligonucleotides as primers and the pCP20 plasmid (Datsenko and Wanner, 2000) as a template.

PCR primers: -FLP 1-D (SEQ ID N° 11): 5′- aaaaggatccaaaaggagggattaaaatgccacaatttggtatattatgtaaaacaccacct-3 ′ -FLP 1-R (SEQ ID N° 12): 5′ -aaatggcgccgcgtacttatatgcgtctatttatgtaggatgaaaggta-3 ′

FLP1-D has a 5′ extension including a BamHI site and the RBS sequence of thl.

FLP 1-R introduced an SfoI site in 3′ of the PCR product.

The PCR product was digested by BamHI and SfoI and directly cloned into the pSOS95 expression vector that had been digested with the same enzymes, giving the pEX-FLP1 (6281pb) plasmid.

The SalI fragment (1585pb) of pEX-FLP1 containing the FLP1 expression cassette was cloned at the SalI site of pCONS2.1 to obtain the pCLF1 (4878pb) plasmid (FIG. 5).

Example 2 Deletion of the cac1502 Gene Encoding the cac824I Restriction Enzyme in Clostridium Acetobutylicum

See FIG. 6 for a schematic representation of the method.

Two DNA fragments surrounding the Cac824I encoding gene (CAC1502) were PCR amplified with Pwo DNA polymerase using total DNA from C. acetobutylicum as template and two specific pairs of oligonucleotides as primers (see table 2). Using pairs of primers CAC 1B-CAC 2 and CAC 3-CAC 4B, 1493 bp and 999 bp DNA fragments were obtained, respectively. Both primers CAC 1B and CAC 4B introduce a BamHI site while primers CAC 2 and CAC 3 have complementary 5′ extended sequences which introduce a StuI site. DNA fragments CAC 1B-CAC 2 and CAC 3-CAC 4B were joined in a PCR fusion experiment with primers CAC 1B and CAC 4B and the resulting fragment was cloned in the pCR4-TOPO-Blunt vector to yield pTOPO: cac 15. At the unique StuI site of pTOPO: cac 15, the 1372 bp StuI fragment of pUC18-FRT-MLS2 harboring the antibiotic resistance MLS^(r) gene with FRT sequences on both sides was introduced. The cac1502 replacement cassette obtained after BamHI digestion of the resulting plasmid was cloned, at the BamHI, into pCons2-1 site to yield the pREPCAC15 plasmid and into pCIP2.1 to yield pCIPCAC15. The pREPCAC15 and pCIPCAC15 plasmids were methylated in vivo and used to transform C. acetobutylicum by electroporation. After selection on Petri plates for clones resistant to erythromycin (40 μg/ml), one colony of each transformants was cultured for 24 hours in liquid synthetic medium with erythromycin at 40 μg/ml and then subcultured in liquid 2YTG medium without antibiotic. Appropriate dilutions were plated on reinforced Clostridium agar (RCA) with erythromycin at 40 μg/ml. To select integrants having lost pREPCAC15 or pCIPCAC15 vectors, erythromycin resistant clones were replica plated on both RCA with erythromycin at 40 μg/ml and RCA with thiamphenicol at 50 μg/ml. While several colonies with the desired phenotype were obtained with the pREPCAC15 transformants, no such colonies were obtained with the pCIPCAC15 transformants. This demonstrates that the θ mechanism of replication of pCIPCAC15 is less favorable in promoting double crossover in C. acetobutylicum than a rolling circle mechanism. The genotype of clones resistant to erythromycin and sensitive to thiamphenicol was checked by PCR analysis (with primers CAC 0 and CAC 5 located outside of the CAC15 replacement cassette and primers CAC D and CAC R located inside of cac1502). Δcac15::mls^(R) strains which have lost pREPCAC15 were isolated.

A Δcac15::mls^(R) strain was transformed with the pCLF1 vector expressing the FLP1 gene encoding the Flp recombinase from S. cerevisiae. After transformation and selection for resistance to thiamphenicol (50 μg/ml) on Petri plates, one colony was cultured on synthetic liquid medium with thiamphenicol at 50 μg/ml and appropriate dilutions were plated on RCA with thiamphenicol at 50 μg/ml. Thiamphenicol resistant clones were replica plated on both RCA with erythromycin at 40 μg/ml and RCA with thiamphenicol at 50 μg/ml. The genotype of clones with erythromycin sensitivity and thiamphenicol resistance was checked by PCR analysis with primers CAC 0 and CAC 5B. Two successive 24 hours cultures of the Δcac15 strain with erythromycin sensitivity and thiamphenicol resistance were carried out in order to loose pCLF1. The Δcac15 strain which has lost pCLF1 was isolated according to its sensitivity to both erythromycin and thiamphenicol. The strain was called C. acetobutylicum MGCΔcac15.

TABLE 2 Name SEQ ID N° Primer sequences CAC 1B 13 aaaggatccatgcacactcataaatttactgtaggaagtctg′ CAC 2 14 ggggaggcctaaaaaggggggtcccaaataatatttgccatagtaaccacc CAC 3 15 ccccctttttaggcctcccctcgaacttattagaatgattaagattccgg CAC 4B 16 aaaggatcctcattaaatttcctccattttaagcctgtc CAC 0 17 gtgatataattttcctttaaatggaggaggatctg CAC 5B 18 gccgttaatagacattataattccattggc CAC D 19 gaattcttaaaaatatttggatcattaagcgg CAC R 20 gttgtattggaatctttgttattatttctccc

Example 3 Deletion of the upp Gene Encoding the Uracil Phosphoribosyl-Transferase in Clostridium acetobutylicum Δcac15

Two DNA fragments upstream and downstream of upp (CAC2879) were PCR amplified with Pwo DNA polymerase using total DNA from C. acetobutylicum as template and two specific pairs of olignonucleotides as primers (see table 3). With the primer pairs UPP 1-UPP 2 and UPP 3-UPP 4, 1103 bp and 1105 bp DNA fragments were obtained, respectively. Both primers UPP 1 and UPP 4 introduce a BamHI site, while primers UPP 2 and UPP 3 have 5′ extended sequences which introduce a StuI site. DNA fragments UPP 1-UPP 2 and UPP 3-UPP 4 were joined in a PCR fusion experiment with primers UPP 1 and UPP 4 and the resulting fragment was cloned in pCR4-TOPO-Blunt to yield pTOPO:upp. At the unique StuI site of pTOPO:upp, the 1372 bp StuI fragment of pUC18-FRT-MLS2 harboring the antibiotic resistance MLS^(r) gene with FRT sequences on both sides was introduced. The upp replacement cassette obtained after BamHI digestion of the resulting plasmid was cloned into pCons2-1 at the BamHI site to yield the pREPUPP plasmid. The plasmid pREPUPP was used to transform C. acetobutylicum MGCΔcac15 strain by electroporation without previous in vivo methylation. After selection on Petri plates for clones resistant to erythromycin (40 μg/ml), one colony was cultured for 24 hours in liquid synthetic medium with erythromycin at 40 μg/ml and then subcultured in liquid 2YTG medium without antibiotic. Appropriate dilutions were plated on RCA with erythromycin at 40 μg/ml. To select integrants having lost the pREPUPP vector, erythromycin resistant clones were replica plated on both RCA with erythromycin at 40 μg/ml and RCA with thiamphenicol at 50 μg/ml. The genotype of clones resistant to erythromycin and sensitive to thiamphenicol was checked by PCR analysis (with primers UPP 0 and UPP 5 located outside of the UPP replacement cassette and primers UPP D and UPP R located inside of UPP). The Δcac15Δupp::mls^(R) strain that has lost pREPUPP was isolated. The previous cac1502 deletion was confirmed as previously described in the first example. The Δcac15Δupp::mls^(R) strain is resistant to 400 μM 5-FU compared to 50 μM for the Δcac15 strain.

The Δcac15Δupp::mls^(R) strain was transformed with the pCLF1 vector expressing the FLP1 gene encoding the Flp recombinase from S. cerevisiae. After transformation and selection for resistance to thiamphenicol (50 μg/ml) on Petri plates, one colony was cultured in synthetic liquid medium with thiamphenicol at 50 μg/ml and appropriate dilutions were plated on RCA with thiamphenicol at 50 μg/ml. Thiamphenicol resistant clones were replica plated on both RCA with erythromycin at 40 μg/ml and RCA with thiamphenicol at 50 μg/ml. The genotype of clones with erythromycin sensitivity and thiamphenicol resistance was checked by PCR analysis with primers UPP 0 and UPP 5. Two successive 24 hours cultures of the Δcac15Δupp strain with erythromycin sensitivity and thiamphenicol resistance were carried out in order to lose pCLF1. The Δcac15Δupp strain that has lost pCLF1 was isolated by determining its sensitivity to both erythromycin and thiamphenicol.

TABLE 3 Name SEQ ID N° Primer sequences UPP 1 21 aaaaggatcctcctgatctattaattcttgatgaaccc UPP 2 22 ggggaggcctaaaaagggggattgcataaataaaaagggctgaaaaataaatttcag UPP 3 23 ccccctttttaggcctccccttatttcattcctccattgtattttttttctatttg UPP 4 24 aaaaggatccgctattatgaataggttaaataagtcagctgg UPP 0 25 aatacaagcaaagagaataggctatgtgcc UPP 5 26 aatacaagcaaagagaataggctatgtgcc UPP D 27 ggcatatgaagtaacaagagaaatgcagc UPP R 28 ataatctatctccagcatctccaagacc

Example 4 Deletion of the cac3535 Gene with the Use of upp and 5-Fluorouracil as a Positive Selection for Plasmid Loss

Two DNA fragments upstream and downstream of cac3535 gene encoding a second restriction-modification system of C. acetobutylicum were PCR amplified with Pwo DNA polymerase using total DNA from C. acetobutylicum as template and two specific pairs of olignonucleotides (see table 4). With the primer pairs RM 1-RM 2 and RM 3-RM 4, 1 kbp and 0.9 kbp DNA fragments were obtained, respectively. Both primers RM 1 and RM 4 introduce a BamHI site, while primers RM 2 and RM 3 have complementary 5′ extended sequences that introduce aStuI site. DNA fragments RM 1-RM 2 and RM 3-RM 4 were joined in a PCR fusion experiment using primers RM 1 and RM 4 and the resulting fragment was cloned in pCR4-TOPO-Blunt vector to yield the pTOPO:cac3535 plasmid. At the unique StuI site of pTOPO:cac3535, the 1372 bp StuI fragment of pUC18-FRT-MLS2 harboring the antibiotic resistance MLS^(r) gene with FRT sequences on both sides was introduced. The CAC3535 replacement cassette obtained after BamHI digestion of the resulting plasmid was cloned into pCons::upp at the BamHI site to yield the pREPCAC3535::upp plasmid.

The pREPCAC3535::upp plasmid was used to transform the C. acetobutylicum MGCΔcac15Δupp strain by electroporation. After selection for erythromycin (40 μg/ml) resistant clones on Petri plates, one colony was cultured for 24 hours in liquid synthetic medium with erythromycin at 40 μg/ml and 100 μl of undiluted culture were plated on RCA with erythromycin at 40 μg/ml and 5-FU at 400 μM. Colonies resistant to both erythromycin and 5-FU were replica plated on both RCA with erythromycin at 40 μg/ml and RCA with thiamphenicol at 50 μg/ml to verify that 5-FU resistance is associated with thiamphenicol sensitivity. The genotype of clones resistant to erythromycin and sensitive to thiamphenicol was checked by PCR analysis (with primers RM 0 and RM 5 located outside of the cac3535 replacement cassette and primers RM D and RM R located inside of cac3535 gene). In this way the Δcac15ΔuppΔcac35::mls^(R) strain that has lost pREPCAC3535::upp was isolated.

TABLE 4 Name SEQ ID N° Primer sequences RM 1 29 aaaaggatccgcagctttctggaaggactacggcg RM 2 30 ggggaggcctaaaaagggggcatttacttatggtacggttcacccc RM 3 31 ccccctttttaggcctccccgtctttaaaaagtaatttatcaaaggcatcaaggc RM 4 32 ccccctttttaggcctccccgtctttaaaaagtaatttatcaaaggcatcaaggc RM 0 33 cacattgtcatttataaaagtccctaggg RM 5 34 gtagtaattccaacttcaactcttgccac RM D 35 cttagaatagctgatattgcttgcgg RM R 36 agcatctctcttaatgattctccgg

REFERENCES

-   Bennett G N, Scotcher M C -   BLOCKING SPORULATION BY INHIBITING SPOIIE -   WO2006007530 published on 2006 Jan. 19 -   Applicant: RICE UNIVERSITY (US) -   Biswas I, Gruss A, Ehrlich S D, Maguin E. -   High-efficiency gene inactivation and replacement system for     gram-positive bacteria. J. Bacteriol. 1993 June; 175(11):3628-35. -   Datsenko K A, Wanner B L. -   One-step inactivation of chromosomal genes in Escherichia coli K-12     using PCR products. Proc Natl Acad Sci USA. 2000 Jun. 6;     97(12):6640-5. -   Fabret C, Ehrlich S D, Noirot P.

A new mutation delivery system for genome-scale approaches in Bacillus subtilis. Mol. Microbiol. 2002 October; 46(1):25-36.

-   Girbal L, Mortier-Barriere I, Raynaud F, Rouanet C, Croux C,     Soucaille P. -   Development of a sensitive gene expression reporter system and an     inducible promoter-repressor system for Clostridium acetobutylicum. -   Appl Environ Microbiol. 2003 August; 69(8):4985-8. -   Green E M, Bennett G N. -   Inactivation of an aldehyde/alcohol dehydrogenase gene from     Clostridium acetobutylicum ATCC 824. -   Appl Biochem Biotechnol. 1996 Spring; 57-58:213-21. -   Green E M, Boynton Z L, Harris L M, Rudolph F B, Papoutsakis E T,     Bennett G N. -   Genetic manipulation of acid formation pathways by gene inactivation     in Clostridium acetobutylicum ATCC 824. -   Microbiology. 1996 August; 142 (Pt 8):2079-86. -   Harris L M, Welker N E, Papoutsakis E T. -   Northern, morphological, and fermentation analysis of spo0A     inactivation and overexpression in Clostridium acetobutylicum ATCC     824. -   J. Bacteriol. 2002 July; 184(13):3586-97. -   Huang I H, Waters M, Grau R R, Sarker M R. -   Disruption of the gene (spo0A) encoding sporulation transcription     factor blocks endospore formation and enterotoxin production in     enterotoxigenic Clostridium perfringens type A. FEMS Microbiol Lett.     2004 Apr. 15; 233(2):233-40. -   Mermelstein L D, Welker N E, Bennett G N, Papoutsakis E T. -   Expression of cloned homologous fermentative genes in Clostridium     acetobutylicum ATCC 824. -   Biotechnology (N Y). 1992 February; 10(2):190-5. -   Ohtani K, Hayashi H, Shimizu T. -   The luxS gene is involved in cell-cell signalling for toxin     production in Clostridium perfringens. -   Mol. Microbiol. 2002 April; 44(1):171-9. -   Reyrat J M, Pelicic V, Gicquel B, Rappuoli R. -   Counterselectable markers: untapped tools for bacterial genetics and     pathogenesis. -   Infect Immun. 1998 September; 66(9):4011-7. Review. -   Sarker M R, Carman R J, McClane B A. -   Inactivation of the gene (cpe) encoding Clostridium perfringens     enterotoxin eliminates the ability of two cpe-positive C.     perfringens type A human gastrointestinal disease isolates to affect     rabbit ileal loops. -   Mol. Microbiol. 1999 September; 33(5):946-58. Erratum in: Mol     Microbiol 2000 January; 35(1):249. -   Varga J, Stirewalt V L, Melville S B. -   The CcpA protein is necessary for efficient sporulation and     enterotoxin gene (cpe) regulation in Clostridium perfringens. -   J. Bacteriol. 2004 August; 186(16):5221-9. 

The invention claimed is:
 1. A process for the replacement of a target DNA sequence by homologous recombination in Clostridia, wherein the process comprises the following steps: transforming a strain with a vector comprising: an origin of replication permitting its replication in Clostridia, and a replacement cassette comprising a first marker gene surrounded by two sequences homologous to selected regions around the target DNA sequence, allowing the recombination of the cassette, and a second marker gene, selecting strains having integrated in their genome said cassette that express the first marker gene, and selecting strains having eliminated said vector that do not express the second marker gene, wherein the second marker gene is a counter-selectable marker that restores the activity of a non-essential absent or deleted gene and whose presence is lethal in the presence of its cognate substrate, chosen among the list consisting of: genes giving sensitivity to 5-fluoro-uracile (5-FU), genes giving sensitivity to gamma-glutamyl hydrazide (GBS), genes giving sensitivity to 8-aza-2,6-diaminopurine (8ADP), genes giving sensitivity to sucrose, genes giving sensitivity to streptomycin, genes giving sensitivity to fusaric acid, pheS gene, thyA gene, lacY gene, gata-I gene, and ccdB gene.
 2. The process according to claim 1, wherein said vector comprises a third marker that permits a negative selection of strains having eliminated said vector.
 3. The process according to claim 1, wherein the vector is eliminated by digestion with endonucleases after the recombination event has occurred.
 4. The process according to claim 3 wherein the vector harbors DNA sequences that are recognized by restriction endonucleases and that are at the same time absent trout the genome of the used Clostridia strain.
 5. The process according to claim 3, wherein the used Clostridia strain harbors on its genome at least one endonuclease encoding gene, specific for restriction sites present in the vector, optionally expressed under the control of an inducible promoter.
 6. The process according to claim 1, wherein the first marker gene is an antibiotic resistance gene.
 7. The process according to claim 1, wherein the first marker gene is surrounded by two recombinase target sites.
 8. The process according to claim 7, wherein the first marker gene is eliminated by the action of a recombinase after the homologous recombination event has occurred.
 9. The process according to claim 8, wherein said recombinase is expressed by a gene carried by a second vector introduced into the strain.
 10. The process according to claim 8, wherein said recombinase target sites are FRT sequences, and said recombinase is the FLP recombinase.
 11. The process according to claim 1, wherein the Clostridia to be transformed are deleted for upp gene.
 12. The process according to claim 1, wherein the Clostridia strains are chosen among Clostridium acetobutylicum, Clostridium bejeirinckii, Clostridium saccharoperbutylacetonicum, Clostridium butylicum, Clostridium butyricum, Clostridium perfringens, Clostridium tetani, Clostridium sporogenes, Clostridium thermocellum, Clostridium saccharolyticum (now Thermoanaerobacter saccharolyticum), Clostridium thermosulfurogenes (now Thermoanaerobacter thermosulfurigenes), Clostridium thermohydrosulfuricum (now Thermoanaerobacter ethanolicus).
 13. The process according to claim 12 wherein the Clostridia strain is Clostridium acetobutylicum.
 14. The process according to claim 13, wherein the Clostridium acetobutylicum strain to be transformed is deleted for the gene cac1502 encoding the restriction endonuclease Cac 824I.
 15. The process according to claim 13, wherein the Clostridium acetobutylicum strain to be transformed is deleted for the gene upp.
 16. The process according to claim 13, wherein the Clostridium acetobutylicum strain to be transformed is deleted for the gene cac1502 encoding the restriction endonuclease Cac 824I, and for the gene upp.
 17. The process for replacement of two or more target genes by homologous recombination in a same Clostridia strain, wherein the process according to claim 1 is performed successively two or more times. 